The present invention relates generally to display systems, and more particularly to aberration correction for cathode ray tube (CRT) images.
Geometric distortion, defocusing, astigmatism and brightness disparity are commonly found on images formed on CRT displays. In many cases, such aberrations are tolerated by the viewer in small amounts. In the case of high resolution graphic and projection displays, however, such aberrations detract noticeably from the quality of the image and its usefulness to the viewer. Aberrations are especially noticeable in the case of quasi-static displays, such as those encountered in connection with military charts and maps in command and control applications.
Methods to correct aberrations provide dynamic modulation of the appropriate CRT electrodes or deflection means by correction signals generated from a prestored set of digital values, and by correction signals derived by analog methods based on CRT deflection signals. Both of these methods have limitations that make them less than fully effective. The digital method has the advantage of being able to cope with nonlinear characteristics, but fails to connect correction data and CRT image position with certainty, especially when various line rates and aspect ratios must be accepted by the display equipment. The analog method fails to cope with the nonlinear characteristics common to production model CRTs.
Compensation devices for display monitors and particularly for CRTs are well known in the art. In the case of conventional analog compensation circuits, electrical signals representative of CRT deflection signals are provided to analog signal processing circuits to generate related correction signal waveforms. The operator adjusts potentials and components of the analog circuits to achieve a suitable correction. The analog correction method is commonly used for focus and astigmatism correction. However, such analog techniques cannot accommodate pixel by pixel compensation as is required in many types of modern CRT based systems.
Conventional compensation devices, such as brightness compensation devices, are often implemented with operator controls and some, such as convergence compensation devices may be implemented with internal maintenance controls. However, no conventional compensation device has been built that automatically compensates for scan rate and aspect ratio changes without modifying the equipment.
Accordingly, it would be desirable to have a circuit for a CRT monitor that adaptively corrects for convergence, focus, linearity, distortion, astigmatism, brightness variations and which adapts to changes in aspect ratio and scanning rate. It would also be advantageous to have a system that provides a link between correction data address and image element position that is independent of image aspect ratios and scanning rates to facilitate instantaneous automatic correction for display equipment operating in an environment of differing scanning standards and aspect ratios.